origin/upstream holds upstream code and origin/pristine-tar holds small binary deltas. Using these two branches it is possible to regenerate upsteam tarball with identical checksum. You can look at them and see what they store:

$ git checkout origin/pristine-tar
$ ls

You'll get a warning after first command, it's fine that's expected. Anyways let's go back to building the package.

$ git checkout master

First you need to do a small setup of git-buildpackage

$ cp /etc/git-buildpackage/gbp.conf .git/
$ nano .git/gbp.conf

This file is commented and mostly self-explanatory. For now just comment out one line:

#pristine-tar = True
change to
pristine-tar = True

and save. This tells git-buildpackage that it can use pristine-tar to generate orig.tar.gz. So now if you envoke this:

$ git buildpackage

It should start building master branch in the ../build-area. Sit back, relax and watch it do it's magic. If on the other hand you want to build your whizbang branch you have two options, either run:

$ git buildpackage --git-debian-branch=whizbang

or edit one line in the .git/gbp.conf:

#debian-branch = master
change to
debian-branch = whizbang

and then run git buildpackage

For more information about git-buildpackage see it's documentation, which is located here /usr/share/doc/git-buildpackage/manual-html/index.html.